18.5.1 Observational models

A performance evaluation which includes comparisons between modeled and observed

ratios of indicator species carries with it a large potential advantage. For ozone, measurements of

certain “indicator species ratios” are a potentially useful way to assess whether local ozone

formation is VOC- or NOx-limited at any particular point in space and time as well as, help reveal

whether the model is correctly predicting the sensitivity of ozone to VOC and/or NOx controls

(e.g., comparisons between modeled and observed ratios: O3/NOy, O3/HNO3) (Sillman, 1995,

1997, 1998, and 2002; Lu and Chang, 1998; Pun and Seigneur, 1999). For PM, such a

comparison may reveal whether the model is predicting sensitivity of secondary components of

PM2.5 to changes in SO2, NH3, VOC and/or NOx controls correctly (Ansari and Pandis, 1998;

Blanchard et al., 2000; Pun and Seigneur, 2001). If a model accurately predicts observed ratios of

indicator species, then one can conclude with additional confidence that the predicted change in

future year ozone or PM may be accurate. One precaution with respect to the use of indicator

species is that there may be a range of observed ratios for which the preferred direction of control

is not clear. When this occurs, agreement between predictions and observations does not

necessarily imply that the response to controls, as predicted by the model is correct (especially for

secondary particulate matter to changes in precursors). If a model predicts observed ratios of

indicator species such that observed and predicted ratios fall within the same range of ratios, this

provides some confidence that the predicted change in particulate matter may be accurate. A

second precaution is that application of this method often requires more measurements than are

commonly made. In some cases, it may be difficult to achieve the required precision with current

routine monitoring. Finally, much of the work done to date with indicator species has focused on

peak hourly concentrations of ozone. Despite these precautions, comparing predicted and

observed ratios of indicator species provides a means of assessing a model's ability to accurately

characterize the sensitivity of predicted ozone and predicted secondary components of PM2.5 to

changes in precursors.

 

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